Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment (UE), etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
A multiple input multiple output (MIMO) scheme is used as a technique for supporting a reliable high-speed data service. The MIMO scheme uses multiple transmit (Tx) antennas and multiple receive (Rx) antennas to improve data transmission/reception efficiency. Examples of the MIMO scheme include spatial multiplexing, transmit diversity, beamforming, etc.
A MIMO channel matrix is formed by multiple Rx antennas and multiple Tx antennas. A rank can be obtained from the MIMO channel matrix. The rank is the number of spatial layers. The rank may also be defined as the number of spatial streams that can be simultaneously transmitted by a transmitter. The rank is also referred to as a spatial multiplexing rate. If the number of Tx antennas is Nt and the number of Rx antennas is Nr, a rank R satisfies R≦min{Nt, Nr}.
A wireless communication system requires a signal known to both a transmitter and a receiver to perform channel measurement, information demodulation, or the like. The signal known to both the transmitter and the receiver is referred to as a reference signal (RS). The RS may also be referred to as a pilot.
The receiver may estimate a channel between the transmitter and the receiver by using the RS, and may demodulate information by using the estimated channel. For example, when a UE receives an RS transmitted by a base station, the user equipment may measure a channel by using the RS, and may feed back channel state information to the base station. A signal transmitted from the transmitter experiences a channel corresponding to each Tx antenna or each spatial layer, and thus the RS may be transmitted for each Tx antenna or each spatial layer.
Meanwhile, there is an ongoing standardization effort for an international mobile telecommunication-advanced (IMT-A) system in the international telecommunication union (ITU) as a next generation (i.e., post 3rd generation) mobile communication system. The IMT-A system aims at the support of an Internal protocol (IP)-based multimedia seamless service by using a high-speed data transfer rate of 1 gigabits per second (Gbps) in a downlink and 500 megabits per second (Mbps) in an uplink. A 3rd generation partnership project (3GPP) is considering a 3GPP long term evolution-advanced (LTE-A) system as a candidate technique for the IMT-A system.
An LTE system supports up to 4 Tx antennas in downlink transmission, whereas the LTE-A system supports up to 8 Tx antennas in downlink transmission. However, a UE to which the LTE system is applied (hereinafter, an LTE UE) and a UE to which the LTE-A system is applied (hereinafter, an LTE-A UE) can coexist in a cell. Therefore, the LTE-A system needs to be designed to support both the LTE UE and the LTE-A UE. In addition, various transmission schemes may exist for downlink transmission. Examples of the transmission scheme include a single-antenna scheme, a MIMO scheme, etc. Examples of the MIMO scheme include a transmit diversity scheme, a closed-loop spatial multiplexing scheme, an open-loop spatial multiplexing scheme, and a MU-MIMO scheme.
As such, when the maximum number of supported Tx antennas differs and when UEs to which various transmission schemes are applied coexist in a system, there is a need to provide a transmission apparatus and method capable of transmitting a reference signal optimized as much as possible for each UE.